Particulate dominance of organic carbon mobilization from thaw slumps on the Peel Plateau, NT: Quantification and implications for stream systems and permafrost carbon release

Abstract

Climate change is increasing the frequency and intensity of thermokarst, and accelerating the delivery of terrestrial organic material from previously sequestered sources to aquatic systems, where it is subject to further biochemical alteration. Rapid climate change in the glacially conditioned ice-rich and ice-marginal terrain of the Peel Plateau, western Canada, is accelerating thaw-driven mass wasting in the form of retrogressive thaw slumps, which are rapidly increasing in area, volume and thickness of permafrost thawed. Despite major perturbation of downstream sedimentary and geochemical fluxes, few studies have examined changes in flux and composition of particulate organic carbon (POC) in streams and rivers as a result of permafrost thaw. Here we show that the orders of magnitude increase in total organic carbon, nitrogen, and phosphorus mobilized to streams from thaw slumps on the Peel Plateau is almost entirely due to POC and associated particulate nitrogen and phosphorus release. Slump-mobilized POC is compositionally distinct from its dissolved counterpart and appears to contain relatively greater amounts of degraded organic matter, as inferred from base-extracted fluorescence of particulate organic matter. Thus, slump-mobilized POC is potentially more recalcitrant than POC present in non-slump affected stream networks. Furthermore a substantial portion of POC mobilized from thaw slumps will be constrained within primary sediment stores in valley bottoms, where net accumulation is currently exceeding net erosion, resulting in century to millennial scale sequestration of thermokarst-mobilized POC. This study highlights the pressing need for better knowledge of sedimentary cascades, mobilization, and storage reservoirs in slump-affected streams, and baseline assessments of the biodegradability of POC and cycling of particulate nutrients within a sedimentary cascade framework. Explicit incorporation of POC dynamics into our understanding of land-water carbon mobilization in the face of permafrost thaw is critical for understanding implications of thermokarst for regional carbon cycling and fluvial ecosystems.

Downstream Evolution of Particulate Organic Matter Composition From Permafrost Thaw Slumps

“…Yet, DOC has been shown to be highly biolabile (Littlefair et al, 2017;Littlefair and Tank, 2018). Release of POC from RTS features is orders of magnitude higher than that of DOC, and it differs in its bulk geochemical composition relative to DOC (Shakil et al, 2020), but molecular composition or degradability of this carbon pool is largely unknown.…”

“…Streams affected by RTS features and active layer detachment slides increase slope sediment yields by orders of magnitude Rudy et al, 2017;Shakil et al, 2020). Based on yearly (2016)(2017) volume loss from headwall retreat of two of the larger RTS features on the Peel Plateau, in the Stony Creek watershed (FM2 and FM3), their combined sediment yields were estimated to be 0.9 × 10 5 m 3 km 2 yr −1 (van der Sluijs et al, 2018;Shakil et al, 2020). The high amounts of sediment and OC released from RTS features feed into headwater catchments, potentially affecting entire watersheds (Kokelj et al, , 2017b(Kokelj et al, , 2020.…”

“…There are still large uncertainties in the current knowledge on the characteristics of POC and its decomposability, especially in ice-rich glacigenic terrain where climate history, paleoecology and parent materials may vary (Shakil et al, 2020). Here, we address these knowledge gaps by characterizing the composition and degradation status of POC released from two active RTS sites (FM3 and FM2) over a 12 km-long downstream transect.…”

Emerging dominance of summer rainfall driving High Arctic terrestrial-aquatic connectivity

“…It is likely these thresholds differ with increasing watershed area, location of the disturbance within watersheds, hydro-geomorphological connectivity, differing types of geomorphological permafrost disturbance (e.g., retrogressive thaw slumps, thermokarst, thermo-erosion), variability in permafrost conditions (sporadic, discontinuous, continuous), watershed characteristics (e.g., OM content, channel type, vegetation distribution), and continued changes in climate and hydrology. However, as the magnitude and frequency of permafrost thaw-induced geomorphological disturbance intensifies due to climate change, our results suggest hydrologically-coupled disturbances will become an increasingly important mechanism for delivering POC from terrestrial environments into stream networks 52 .…”

Retrogressive Thaw Slumps on Ice‐Rich Permafrost Under Degradation: Results From a Large‐Scale Laboratory Simulation

“…In close association with the erosion of RTS, small mud flows settling on gentle slopes are observed from melted ice‐wedge network and contribute to the transport of sediments, dissolved elements, and meltwater from the melting of ice wedges (Figure 1a). These RTS liberate both soluble materials and organic carbon accumulated in the sedimentary deposits that are transported by runoff (Shakil et al., 2020; Vonk et al., 2015). The abrupt thawing of ice‐rich permafrost is thus a major issue because of the positive feedback on global climate (Schuur et al., 2008; Turestsky et al, 2020).…”